To produce forged annular pieces, such as sleeves, for example to build nuclear power plant vessels or petrochemical reactors, it is known to use ingots that are solid, which must then be subject to a forging operation including the piercing of an axial hole, or direct casting of ingots comprising a central bore that can be transformed directly in the form of a sleeve.
These two types of ingots in particular differ in terms of the casting conditions, which result in particular in hydrogen contents kept in the molten state and that can have an impact both on the properties of the obtained pieces and the manufacturing conditions.
The solid ingots can be vacuum cast, which allows them to be made from steel that has been degassed during the vacuum casting, to obtain hydrogen content levels guaranteed to be less than 1 ppm.
However, the ingots comprising a central bore are bottom cast in the air. These ingots are cast using metal or molten steel that has been degassed during ladle metallurgy operations, and which in general have a guaranteed hydrogen content below 1.5 ppm. However, during bottom casting, through the passage through the air and the contact with the refractories that constitute the source, the steel regains a hydrogen quantity in the vicinity of 0.3 ppm, and it is therefore difficult to obtain ingots for which it is possible to guarantee, when the steel is in the molten state in the ingot mold, a hydrogen content below 1.8 ppm.
However, for certain applications, and in particular for applications in the field of the construction of nuclear reactors, it is necessary to obtain parts whereof the hydrogen content on the finished pieces is less than 0.8 ppm. Such content levels can be obtained with vacuum cast solid ingots in particular when the pressure in the vacuum cast enclosure is in the vicinity of 0.1 Torr. However, with bottom cast ingots, and in particular ingots comprising a longitudinal bore, this guarantee can only be obtained by subjecting the pieces during forging to a series of long and costly thermal treatments intended in particular to diffuse the oxygen. It therefore results from these differences that although ingots having a longitudinal bore can be forged with a simplified forging process relative to solid ingots, they on the other hand require very long and very expensive degassing treatments that make the process more complicated.
However, although the solid ingots have a low hydrogen content level and therefore do not require degassing treatments, they require a more complicated forging process. In fact, this process must include at least one step intended to produce a central hole that requires several forging and heating operations in furnaces.